This invention is directed to negative-acting photoimageable compositions which are developable in alkaline aqueous solutions. The invention is particularly applicable to primary photoimaging resists, but is applicable, as well, to compositions that are hardenable so as to form solder masks and the like.
It is well known to use as binders in such photoimageable compositions styrene/maleic anhydride (SMA) copolymers which are esterified with various alcohols, e.g., U.S. Pat. No. 5,609,991 the teachings of which are incorporated herein by reference.
U.S. Pat. No. 5,296,334, the teachings of which are incorporated herein by reference, teaches photoimageable compositions which are esterified with a (meth)acrylate functional alcohol, such as hydroxyethylmethacrylate (HEMA). By adding (meth)acrylate functionality to the binder polymer, the binder polymer, along with acrylate functional monomer enters into the photo-initiated cross-linking reaction, thereby increasing the cross-linked density of the exposed resist.
(Meth)acrylate functional binder polymers, such as those taught in U.S. Pat. No. 5,296,334 have proven valuable in secondary photoimageable compositions, such as those used for forming permanent layers, e.g., solder masks. On the other hand, (meth)acrylate functional binder polymers have proved difficult to incorporate into primary photoimaging resists, which are not designed to be permanent coatings. In particular, the increased cross-link density afforded by the (meth)acrylate functionality significantly interferes with the stripping process of printed circuit board fabrication. In fact, the increased (meth)acrylate functionality combined with the high molecular weight and hydrophobic character of the styrene made stripping almost impossible in aqueous NaOH type stripping solutions. Moreover, the ethylene oxide link in the HEMA type monomers created potential problems with water sensitivity during developing, etching and plating processing steps.